U.S. patent application number 12/562605 was filed with the patent office on 2010-04-01 for surface emitting laser and manufacturing method therefor.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Mitsuhiro Ikuta.
Application Number | 20100080258 12/562605 |
Document ID | / |
Family ID | 41401576 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100080258 |
Kind Code |
A1 |
Ikuta; Mitsuhiro |
April 1, 2010 |
SURFACE EMITTING LASER AND MANUFACTURING METHOD THEREFOR
Abstract
Provided is a surface emitting laser or the like capable of
suppressing horizontal misalignment between the surface relief
structure and the current confining structure to make higher the
precision of the alignment, to thereby obtain single transverse
mode characteristics with stability. The surface emitting laser
having a semiconductor layer laminated therein includes: a first
etching region formed by etching a part of the upper mirror; and a
second etching region formed by performing etching from a bottom
portion of the first etching region to a semiconductor layer for
forming a current confining structure, in which a depth of the
second etching region is smaller than a depth of the first etching
region.
Inventors: |
Ikuta; Mitsuhiro;
(Kawasaki-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
1290 Avenue of the Americas
NEW YORK
NY
10104-3800
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
41401576 |
Appl. No.: |
12/562605 |
Filed: |
September 18, 2009 |
Current U.S.
Class: |
372/46.013 ;
257/E33.067; 438/31 |
Current CPC
Class: |
H01S 5/18311 20130101;
H01S 2301/166 20130101; H01S 5/18391 20130101; H01S 5/2086
20130101; H01S 5/209 20130101 |
Class at
Publication: |
372/46.013 ;
438/31; 257/E33.067 |
International
Class: |
H01S 5/00 20060101
H01S005/00; H01L 33/00 20100101 H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2008 |
JP |
2008-247737 |
Claims
1. A surface emitting laser, comprising: a substrate; and a
semiconductor layer laminated on the substrate, the semiconductor
layer including a lower mirror, an active layer, and an upper
mirror, the surface emitting laser further comprising: a first
etching region formed by etching a part of the upper mirror; a
second etching region formed by performing etching from a bottom
portion of the first etching region to a semiconductor layer for
forming a current confining structure, the semiconductor layer
being provided one of in the upper mirror and between the upper
mirror and the active layer, below the first etching region; a
surface relief structure by a stepped structure formed in a light
emitting portion on a front surface side of the upper mirror; and
the current confining structure formed by oxidizing a part of the
semiconductor layer for forming the current confining structure,
wherein a depth of the second etching region is shallower than a
depth of the first etching region.
2. A surface emitting laser according to claim 1, wherein the
semiconductor layer for forming the current confining structure is
formed of Al.sub.xGa.sub.(1-x)As (0.9<x.ltoreq.1).
3. A surface emitting laser according to claim 1, wherein the
second etching region is divided into multiple parts.
4. A surface emitting laser according to claim 1, wherein the depth
of the etching of the second etching region is 500 nm or more and 1
.mu.m or less.
5. A surface emitting laser according to claim 1, wherein the depth
of the etching of the second etching region is 100 nm or more and
500 nm or less.
6. A method of manufacturing a surface emitting laser comprising:
laminating a semiconductor layer on a substrate, the semiconductor
layer including a lower mirror, an active layer, and an upper
mirror; forming a first etching region by etching a part of the
upper mirror from an upper portion of the upper mirror; forming a
resist on the upper mirror following the forming the first etching
region; patterning the resist to form a pattern for forming a
surface relief structure by a stepped structure and for forming a
second etching region as one step; forming the second etching
region, following the patterning, using the pattern, from a bottom
portion of the first etching region to a semiconductor layer for
forming a current confining structure, the semiconductor layer
being provided one of in the upper mirror and between the upper
mirror and the active layer, below the first etching region;
forming the current confining structure by oxidizing, from the
second etching region, a part of the semiconductor layer for
forming the current confining structure; and forming, in a light
emitting portion on a front surface side of the upper mirror, the
surface relief structure by the stepped structure by etching,
following the patterning, using the pattern, a part of the upper
mirror, wherein the second etching region is smaller in depth than
the first etching region.
7. A method of manufacturing a surface emitting laser according to
claim 6, wherein the current confining structure is formed
following the forming the surface relief structure.
8. A method of manufacturing a surface emitting laser according to
claim 6, wherein the surface relief structure is formed following
the forming the current confining structure.
9. A method of manufacturing a surface emitting laser according to
claim 6, wherein the etching in the forming the first etching
region is one of wet etching and dry etching.
10. A method of manufacturing a surface emitting laser according to
claim 6, wherein the etching in the forming the second etching
region is one of wet etching and dry etching.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a surface emitting laser
and a manufacturing method therefor.
[0003] 2. Description of the Related Art
[0004] In a vertical cavity surface emitting laser (VCSEL) which is
a kind of a surface emitting laser, light can be taken out in a
direction perpendicular to a surface of a substrate. Therefore, a
two-dimensional array can be formed with ease just by changing a
mask pattern when a device is formed.
[0005] By parallel treatment using multiple beams emitted from the
two-dimensional array, a higher density and a higher speed are made
possible, and industrial application thereof to various fields such
as optical communications is expected.
[0006] For example, when a surface emitting laser array is used as
an exposure light source of an electro-photographic printer, a
higher density and a higher speed of a printing process using
multiple beams are made possible.
[0007] In such a printing process in electro-photography, because a
stable and minute laser spot needs to be formed on a photosensitive
drum, the laser is required to operate with stability in a single
transverse mode and in a single longitudinal mode.
[0008] Recently, for higher performance of a surface emitting
laser, a method of injecting current using selective oxidation as
described in the following has been developed.
[0009] In the selective oxidation, an AlAs layer or an AlGaAs layer
having a high Al composition ratio, for example, having an Al
composition ratio of 98% is provided in a multilayer reflecting
mirror. By its selective oxidation in a water vapor atmosphere at
high temperature, a current confining structure is formed such that
current is injected only into a region in which the current
injection is necessary.
[0010] However, the above-mentioned selective oxidation for forming
the current confining structure is not desirable from the viewpoint
of operation in the single transverse mode.
[0011] More specifically, a difference in the refractive index
which is larger than necessary is caused due to the existence of
the oxide layer, which in turn causes a high order transverse
mode.
[0012] Measures taken against this include a method of preventing
confinement of the high order transverse mode by making the
diameter of the light emitting region as small as about 3 .mu.m to
attain single transverse mode oscillation. However, in such a
method, because the light emitting region becomes smaller, output
per device is significantly lowered.
[0013] Further, because current is injected into a minute light
emitting region, the current density becomes higher, which is a
cause of increase in device resistance, shortened device life, and
the like.
[0014] In view of the above, conventionally, methods of attaining
single transverse mode oscillation while maintaining a light
emitting region being large to some extent by intentionally
introducing a loss difference between a fundamental transverse mode
and the high order transverse mode are reviewed.
[0015] As one of such methods, H. J. Unold et al., Electronics
Letters, Vol. 35, No. 16 (1999) discloses a method of making a high
order transverse mode loss larger than a fundamental transverse
mode loss by forming a stepped structure on a light emitting
surface of a surface emitting laser device.
[0016] FIG. 5 is a schematic sectional view of a surface emitting
laser in which a surface relief structure is formed according to
the method.
[0017] It is to be noted that a structure having a step in a light
emitting region of a light emitting surface of a reflecting mirror
as described above is hereinafter referred to as a surface relief
structure.
[0018] By the way, when a loss difference is given to respective
optical modes of a VCSEL using the surface relief structure,
horizontal alignment between the surface relief structure and the
current confining structure is important.
[0019] More specifically, when only fundamental transverse mode
oscillation is required, the amount of horizontal misalignment
between the center of a current confining aperture and the center
of a surface relief structure is, for example, preferably 1 .mu.m
or less, and more preferably 0.5 .mu.m or less.
[0020] This is because, if the centers are misaligned, an
unnecessary loss is given to the mode in which the oscillation is
required (fundamental transverse mode in this case), or, a
necessary loss can not be given to the mode in which the
oscillation is not required (high order transverse mode).
[0021] In H. J. Unold et al., Electronics Letters, Vol. 35, No. 16
(1999), as such a method of forming the surface relief structure
and the current confining structure with the surface relief
structure and the current confining structure being aligned with
each other, a method called a self-alignment process is
disclosed.
[0022] This method is characterized in that the positioning and
patterning of a surface relief structure and a mesa structure are
carried out at the same time.
[0023] By etching the mesa, a side wall of a selective oxidation
layer is exposed, from which the selective oxidation layer is
oxidized, to thereby form the current confining structure.
[0024] Therefore, the horizontal alignment between the surface
relief structure and the current confining structure is performed
automatically.
[0025] FIGS. 6A to 6E are schematic views for describing the
self-alignment process disclosed in H. J. Unold et al., Electronics
Letters, Vol. 35, No. 16 (1999). FIGS. 6A to 6E describe a
self-alignment process flow. As illustrated in FIG. 6A, a first
resist 410 is applied to an upper mirror 114 of a wafer for a
VCSEL, and the resist 410 is patterned at the same time in the
shape of the surface relief structure and in the shape of the mesa
structure. Here, a convex surface relief is illustrated.
[0026] Then, as illustrated in FIG. 6B, dry etching of the
semiconductor is performed with the patterned resist 410 being used
as a mask. The etching forms a surface relief structure 150.
[0027] Then, as illustrated in FIG. 6C, a second resist 420 is
applied and patterned so as to protect the surface relief structure
150.
[0028] Then, as illustrated in FIG. 6D, wet etching is performed so
as to form the mesa structure, and a high-Al-composition-ratio
layer 115 is exposed at a side wall of the mesa.
[0029] Then, as illustrated in FIG. 6E, the resists 410 and 420 are
removed and the high-Al-composition-ratio layer 115 is selectively
oxidized to form a current confining structure 116.
[0030] From hereon, according to a standard process, an electrode
is connected to the device to complete a VCSEL device.
[0031] By the way, in order to expose the selective oxidation layer
by etching the mesa structure using the above-mentioned
conventional self-alignment process such that the selective
oxidation layer can be oxidized, the depth of the etching is
required to be on the order of 3 to 4 .mu.m.
[0032] In such deep etching, it is difficult to expose the side
wall of the selective oxidation layer according to the position of
the pattern of the mesa structure formed at the same time with the
pattern of the surface relief structure. The reason is described in
the following.
[0033] For example, when a deep mesa structure as described above
is formed by wet etching, there are problems including difficulty
in forming the mesa structure so as to be precisely vertical and
liability to crystal orientation dependence of the
semiconductor.
[0034] Further, when the deep mesa structure as described above is
formed by dry etching, the resistance of the resist to the dry
etching is low.
[0035] Therefore, there is a problem in that, because edge portions
of the etching mask are damaged and pull back, the mesa structure
can not be formed with high precision.
[0036] For those reasons, there is a possibility that the position
at which the oxidation of the high-Al-composition-ratio layer
starts (the position of the side wall exposed by the etching) is
misaligned with the patterning position of the mesa structure.
[0037] In that case, the positions at which the oxidation starts
are misaligned with the position of the patterned mask, and
consequently, the size and the position of the current confining
structure deviate from the size and the position that the current
confining structure should have and become unstable.
[0038] As a result, as exemplarily illustrated in FIG. 7, there is
a possibility that the surface relief structure and the current
confining structure are not necessarily aligned with each other
such that an effective single transverse mode VCSEL can not be
obtained.
SUMMARY OF THE INVENTION
[0039] The present invention has been made in view of the
above-mentioned problems, and an object of the present invention is
to provide a surface emitting laser and a manufacturing method
therefor which can suppress horizontal misalignment between a
surface relief structure and a current confining structure to
achieve higher precision of alignment and which can obtain single
transverse mode characteristics with stability.
[0040] According to the present invention, the surface emitting
laser and the manufacturing method therefor can be materialized
which can suppress the horizontal misalignment between the surface
relief structure and the current confining structure to achieve
higher precision of the alignment and which can obtain single
transverse mode characteristics with stability.
[0041] According to the present invention, the horizontal
misalignment between the surface relief structure and the current
confining structure due to a stepped structure formed for
controlling the transverse mode can be suppressed to make higher
the precision of the alignment. This is based on the following
findings of the inventor(s) of the present invention.
[0042] As described above, when a deep mesa structure is etched, if
the conventional self-alignment process is used and all the
patterning is performed at the same time, it is difficult to form
the mesa structure with high precision.
[0043] Therefore, according to the present invention, the mesa
structure is not etched at a time, and the process of etching the
mesa structure is divided into a first stage etching process of
forming a first etching region and a second stage etching process
of forming a second etching region.
[0044] In the second stage etching process, the above-mentioned
self-alignment process is used to perform shallower etching
compared with the etching of the first etching region in the first
stage. Etching at least to a semiconductor layer for forming the
current confining structure is performed to expose the
semiconductor layer.
[0045] By dividing the etching process into the two stages, a depth
of the etching necessary for exposing the semiconductor layer for
the current confining structure can be made to be shallower than a
depth of the conventional etching performed only once for exposing
the semiconductor layer for the current confining structure.
[0046] Therefore, by etching the second etching region in the
above-mentioned way, a position of a side surface of the exposed
semiconductor layer for forming the current confining structure can
be made nearer to a position of a predetermined patterning aligned
with the surface relief structure.
[0047] This makes it possible to make higher the precision of the
alignment between the surface relief structure and the current
confining structure.
[0048] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] FIG. 1 is a schematic sectional view illustrating an
exemplary structure of a surface emitting laser according to
Embodiment 1 of the present invention.
[0050] FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H, 2I, 2J, and 2K are
schematic views illustrating an exemplary method of manufacturing a
surface emitting laser according to Embodiment 2 of the present
invention.
[0051] FIG. 3 illustrates a layer structure for describing a
specific example of using wet etching when a first etching region
and a second etching region are etched in Embodiment 2 of the
present invention.
[0052] FIGS. 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, and 4I are schematic
views illustrating an exemplary method of manufacturing a surface
emitting laser according to Embodiment 3 of the present
invention.
[0053] FIG. 5 is a schematic sectional view of a surface emitting
laser having a surface relief structure formed thereon according to
a conventional method disclosed in H. J. Unold et al., Electronics
Letters, Vol. 35, No. 16 (1999).
[0054] FIGS. 6A, 6B, 6C, 6D, and 6E are schematic views
illustrating a self-alignment process in Unold et al, Electronics
Letters, Vol. 35, No. 16 (1999).
[0055] FIG. 7 is a schematic view illustrating a conventional case
in which the surface relief structure and a current confining
structure are misaligned.
DESCRIPTION OF THE EMBODIMENTS
[0056] Embodiments of the present invention are described in the
following.
Embodiment 1
[0057] As Embodiment 1, an exemplary structure of a surface
emitting laser in which a semiconductor layer including a lower
mirror, an active layer, and an upper mirror is laminated on a
substrate according to the present invention is described.
[0058] FIG. 1 is a schematic sectional view illustrating an
exemplary structure of a vertical cavity surface emitting laser
(hereinafter, referred to as a surface emitting laser) of this
embodiment.
[0059] As illustrated in FIG. 1, in a surface emitting laser 100 of
this embodiment, a lower mirror 106, a lower spacer layer 108, an
active layer 110, an upper spacer layer 112, and an upper mirror
114 are laminated on a substrate 104.
[0060] A semiconductor layer which is a high-Al-composition-ratio
layer 115 is provided in the upper mirror 114 or between the upper
mirror 114 and the active layer 110. A part of the
high-Al-composition-ratio layer 115 is oxidized to form a current
confining structure 116.
[0061] The high-Al-composition-ratio layer 115 can be formed of
materials of Al.sub.xGa.sub.(1-x)As (0.9<x.ltoreq.1) or
AlAs.
[0062] A mesa structure is formed in the surface emitting laser 100
by a first etching region 170 which forms a part of the region on a
front surface side of the upper mirror 114.
[0063] The mesa structure formed by the first etching region 170 is
above the high-Al-composition-ratio layer 115.
[0064] Further, a second etching region 172 is provided below the
first etching region 170.
[0065] The second etching region 172 forming the mesa structure
ranges from below the first etching region 170 to the
high-Al-composition-ratio layer 115 in the upper mirror 114 or
between the upper mirror 114 and the active layer 110 at least
under the first etching region 170.
[0066] The depth of the second etching region 172 is formed by
etching shallower than the depth of the first etching region
170.
[0067] Further, in the second etching region 172, a side wall of
the high-Al-composition-ratio layer 115 is exposed, and, as
described above, a part of the high-Al-composition-ratio layer 115
is oxidized from the side wall to form the current confining
structure 116 for spatially restricting current injection into the
active layer 110.
[0068] Further, a surface relief structure 150 formed by a stepped
structure for controlling a transverse mode and the current
confining structure 116 are horizontally aligned.
[0069] More specifically, the surface relief structure 150 is
provided over the upper mirror 114 so as to be at the center of the
mesa structure.
[0070] The surface relief structure 150 and the second etching
region 172 are aligned in the direction of a surface of the
substrate 104.
[0071] For example, here, the surface relief structure 150 is a
concentric convex relief structure and is designed such that the
reflectivity in a center portion of the upper mirror 114 is
higher.
[0072] Further, the center of the surface relief structure 150 and
the center of an aperture in the current confining structure 116
are vertically aligned.
[0073] An insulating film 124 is provided on a part of an upper
surface and the side wall of the mesa structure.
[0074] A contact layer (not shown) is provided on an uppermost
portion of the upper mirror 114, and an upper electrode 126 is
connected to a part of the contact layer.
[0075] An opening for taking out light is provided in the upper
electrode 126.
[0076] Further, a lower electrode 102 is connected to a rear side
of the substrate 104.
[0077] The structure of the surface emitting laser 100 of this
embodiment is as described above. By applying predetermined voltage
between the upper electrode 126 and the lower electrode 102, the
active layer 110 emits light, the surface emitting laser 100
oscillates, and laser light is emitted through the opening in the
upper electrode.
[0078] As described above, the etching process is divided into two
stages and the depth of the second stage etching is shallower than
the depth of the first stage etching. By the second stage etching,
the semiconductor layer for forming the current confining structure
is exposed. This makes it possible to make higher precision of the
alignment between the surface relief structure and the current
confining structure, and thus, a surface emitting laser which can
obtain single transverse mode characteristics with stability can be
provided.
[0079] By the way, when, as the high-Al-composition-ratio layer, an
AlGaAs layer having a high Al composition ratio, in particular, an
AlAs layer, is used and is selectively oxidized, the speed of the
oxidation depends on the crystal orientation. For example, the
speed of the selective oxidation in a surface along a (100) axis is
higher than the speed of the selective oxidation in a surface along
a (110) axis. In this case, when the selective oxidation of the
high-Al-composition-ratio layer starts from the side surface of the
circular mesa, the shape of the aperture in the current confining
structure 116 is quadrangle.
[0080] Further, when a normally used (100) surface substrate is not
used but a tilted substrate having an off-angle of 5.degree. or
more, for example, 10.degree., from the (100) surface is used, the
quadrangular shape is further distorted.
[0081] Here, from the viewpoint of reliability, it is preferred
that the current confining structure is circular. When a circular
current confining structure is required to be obtained, the
distance from the center portion to the side wall of the
high-Al-composition-ratio layer may be determined taking into
consideration the difference in the speed of the oxidation in the
directions of the respective surfaces of the
high-Al-composition-ratio layer. In this case, if the position at
which the oxidation of the high-Al-composition-ratio layer starts
is misaligned with the position of the patterning, it is difficult
to form the aperture in the current confining structure such that
its shape is circular.
[0082] However, in the surface emitting laser formed by etching in
the two stages as described above, the position at which the
oxidation starts and the position of the patterning can be aligned
with high precision, and hence the aperture can be formed such that
its shape is circular. Therefore, the aperture in the current
confining structure and the surface relief structure can be aligned
with high precision, and thus, a surface emitting laser which can
obtain single transverse mode characteristics with stability can be
provided.
Embodiment 2
[0083] Next, as Embodiment 2, an exemplary method of manufacturing
a surface emitting laser in which the semiconductor layer is
laminated on a substrate according to the present invention to form
a surface emitting laser is described.
[0084] FIGS. 2A to 2K are schematic views illustrating the
exemplary method of manufacturing the surface emitting laser
according to this embodiment.
[0085] FIGS. 2A to 2K illustrate manufacturing steps of the surface
emitting laser according to this embodiment.
[0086] In FIGS. 2A to 2K, like reference numerals designate like or
identical members illustrated in FIG. 1 in Embodiment 1.
[0087] In the method of manufacturing the surface emitting laser
according to this embodiment, first, in a step illustrated in FIG.
2A, the layers from the lower mirror 106 to the upper mirror 114
are made to grow in the stated order on an n-type GaAs substrate
104.
[0088] For example, the lower mirror 106, the lower spacer layer
108, and the active layer 110 are made to grow by metal organic
chemical vapor deposition (MOCVD).
[0089] Further, the upper spacer layer 112, the
high-Al-composition-ratio layer 115, and the upper mirror 114 are
made to grow on the active layer 110.
[0090] The active layer 110 has a gain at wavelength .lamda.
wherein .lamda. is an oscillation wavelength of the surface
emitting laser 100. .lamda. is, for example, 680 nm.
[0091] More specifically, for example, a GaAs substrate is used as
the substrate 104. The GaAs substrate used is a tilted substrate
with a principal surface thereof being tilted 5 degrees or more
from the (100) surface.
[0092] The lower mirror 106 is a distributed bragg reflector (DBR)
made of sixty pairs of n-type AlAs layers and
Al.sub.0.5Ga.sub.0.5As layers and the optical thickness of each of
the layers is .lamda./4.
[0093] The lower spacer layer 108 is an n-type AlGalnP layer and
the active layer 110 has multiple quantum well structures of
GalnP/AlGalnP.
[0094] Further, the upper spacer layer 112 is a p-type AlInP layer
and the upper mirror 114 is a DBR made of thirty-eight pairs of
p-type Al.sub.0.9Ga.sub.0.1As layers and Al.sub.0.5Ga.sub.0.5As
layers. The optical thickness of each of the layers is
.lamda./4.
[0095] The thicknesses of the lower spacer layer 108 and the upper
spacer layer 112 are adjusted such that the active layer 110 falls
at a loop of a standing wave which is resonated with a vertical
resonator formed by the lower mirror 106 and the upper mirror
114.
[0096] Further, the high-Al-composition-ratio layer 115 is provided
in the upper mirror 114 (for example, at the first pair counted
from the side of the active layer 110).
[0097] More specifically, the high-Al-composition-ratio layer 115
is formed of Al.sub.0.98Ga.sub.0.02As, and has a thickness of 30
nm.
[0098] An uppermost layer of the upper mirror 114 is a GaAs contact
layer which has a thickness of 20 nm.
[0099] Then, in a step illustrated in FIG. 2B, a first protective
layer 260 is formed. After a first resist 262 is applied, the mesa
structure is patterned.
[0100] The first protective layer 260 is formed of, for example,
SiO.sub.2 or SiN, and has a thickness of, for example, 200 nm.
[0101] The first protective layer 260 is formed by, for example,
plasma chemical vapor deposition (CVD). The diameter of the mesa
structure is, for example, 26 .mu.m.
[0102] Then, in a step illustrated in FIG. 2C, wet etching of the
first protective layer 260 is performed with the first resist 262
being used as a mask.
[0103] In this etching, for example, buffered hydrofluoric acid is
used.
[0104] Further, the upper mirror 114 is etched with the first
resist 262 and the first protective layer 260 being used as a
mask.
[0105] This etching is, for example, dry etching or wet etching,
and, for example, reactive ion etching using SiCl.sub.4/Ar gas.
[0106] In the etching step illustrated in FIG. 2C, a first etching
region in which the etching region does not reach the
high-Al-composition-ratio layer 115 is formed.
[0107] More specifically, in the step illustrated in FIG. 2C, a
region corresponding to the first etching region 170 above the
high-Al-composition-ratio layer 115 in the mesa structure described
in FIG. 1 in Embodiment 1 is etched as the first etching
region.
[0108] Then, in a step illustrated in FIG. 2D, the first resist 262
is removed and a second protective layer 264 is formed so as to
cover the mesa structure.
[0109] The first resist 262 is removed by, for example, asking with
oxygen plasma.
[0110] The second protective layer 264 is formed of, for example,
SiO.sub.2 or SiN, and is formed by plasma CVD.
[0111] The thickness of the second protective layer 264 may be, for
example, 500 nm.
[0112] After that, a second resist 266 is applied, and patterning
for forming the surface relief structure 150 and an etching region
for exposing the side wall of the high-Al-composition-ratio layer
115 is performed.
[0113] The patterning is performed for the purpose of forming the
etching region corresponding to the second etching region 172 which
is further provided under the first etching region 170 described in
FIG. 1 in Embodiment 1 and the depth of which is shallower than the
depth of the first etching region 170.
[0114] The patterning is performed by, for example, projection-type
photolithography using the same mask.
[0115] It is preferred that the photolithography is performed such
that the depth of focus of the second resist 266 is equal to or
deeper than the depth of the first etching region 170.
[0116] The second etching region 172 is formed in the shape of, for
example, a ring surrounding the mesa structure.
[0117] The patterning for forming the second etching region 172 is
performed with regard to the second resist 266 immediately under
the first etching region 170.
[0118] The surface relief structure 150 and the second etching
region 172 are patterned such that their centers are aligned.
[0119] It is to be noted that their centers are not necessarily
required to be aligned with the center of the mesa structure (the
structure surrounded by the first etching region 170).
[0120] Then, in a step illustrated in FIG. 2E, the second
protective layer 264 and the first protective layer 260 are etched
with the second resist 266 being used as a mask.
[0121] In etching the second protective layer 264 and the first
protective layer 260, for example, buffered hydrofluoric acid is
used.
[0122] Then, in a step illustrated in FIG. 2F, the second resist
266 is removed. The second resist 266 is removed by, for example,
acetone.
[0123] After that, an upper portion of the upper mirror 114 is
etched with the second protective layer 264 being used as a mask to
form the surface relief structure 150 in the upper mirror 114. The
upper mirror 114 is etched by wet etching.
[0124] Then, in a step illustrated in FIG. 2G, a third protective
layer 268 is formed so as to cover the mesa structure and the
surface relief structure 150.
[0125] The third protective layer 268 is formed of, for example,
SiO.sub.2 or SiN, and is formed by plasma CVD.
[0126] The thickness of the third protective layer 268 is, for
example, 100 nm.
[0127] Then, a third resist 274 is applied and patterned such that
the third resist 274 covers the etched portion of the surface
relief structure 150 and such that the third resist 274 does not
extend beyond an upper portion of the mesa.
[0128] Then, in a step illustrated in FIG. 2H, the third protective
layer 268 is etched with the third resist 274 being used as a
mask.
[0129] In this etching, for example, buffered hydrofluoric acid is
used. In this case, the second protective layer 264 exists on a
side of a part of the third protective layer 268 which is nearer to
the mesa. The etching time is adjusted so that all of the second
protective layer 264 is not etched away.
[0130] Then, in a step illustrated in FIG. 2I, etching is performed
with the third resist 274 and the second protective layer 264 being
used as a mask to form the second etching region 172.
[0131] The second etching region 172 is an etching region
corresponding to the second etching region 172 which is under the
first etching region 170 described with reference to FIG. 1 in
Embodiment 1 described above and the depth of which is shallower
than the depth of the first etching region 170.
[0132] The etching is dry etching or wet etching.
[0133] In the etching of the second etching region 172, the etching
is performed from the upper mirror 114 under the first etching
region 170 until at least the high-Al-composition-ratio layer 115
is exposed.
[0134] In the step illustrated in FIG. 2I in this embodiment, the
second etching region 172 is etched to the lower spacer layer
108.
[0135] Then, in a step illustrated in FIG. 2J, after the third
resist 274 is removed, a part of the high-Al-composition-ratio
layer 115 is oxidized to form the current confining structure
116.
[0136] The third resist 274 is removed by, for example, acetone.
The high-Al-composition-ratio layer 115 is oxidized by, for
example, heating the high-Al-composition-ratio layer 115 to
400.degree. C. and placing the high-Al-composition-ratio layer 115
in a water vapor atmosphere for 30 minutes.
[0137] The high-Al-composition-ratio layer 115 is oxidized from its
side surface which is exposed to the second etching region 172. The
diameter of the aperture in the current confining structure 116
formed by oxidation is, for example, 6 .mu.m.
[0138] Then, in a step illustrated in FIG. 2K, the third protective
layer 268, the second protective layer 264, and the first
protective layer 260 are removed using, for example, buffered
hydrofluoric acid.
[0139] From hereon, according to a standard process, an electrode
is connected to the device to complete the surface emitting laser
100.
[0140] More specifically, as illustrated in FIG. 1, the insulating
film 124 is formed so as to cover the mesa structure and a part of
the insulating film 124 on the upper portion of the mesa is
removed.
[0141] After that, the upper electrode 126 is formed on the mesa
structure and the insulating film 124 such that the opening is
provided in a light emitting portion (including the surface relief
structure 150), while the lower electrode 102 is formed on a lower
side of the substrate 104.
[0142] As the insulating film 124, for example, an SiO.sub.2 film
at a thickness of 200 nm is formed by, for example, plasma CVD.
[0143] The upper electrode 126 is formed of, for example, Ti/Au,
and is formed by lift-off.
[0144] The lower electrode 102 is formed of, for example, AuGe/Au.
It is to be noted that, if necessary, the surface emitting laser
100 may be annealed at about 300.degree. C. to improve the extent
of contact at an interface between the electrode and the
semiconductor.
[0145] In the manufacturing method described in the above, the
depth of the etching necessary for exposing the
high-Al-composition-ratio layer 115 may be shallower than the depth
in a conventional case.
[0146] More specifically, the depth of the etching of the second
etching region may be 500 nm or more and 1 .mu.m or less, 100 nm or
more and 500 nm or less, or shallower.
[0147] Therefore, the position of the exposed side surface of the
high-Al-composition-ratio layer 115 is made nearer to the position
of the predetermined patterning aligned with the surface relief
structure 150.
[0148] Therefore, a surface emitting laser in which the precision
of the relative alignment between the aperture in the current
confining structure 116 formed by oxidation from the side surface
of the high-Al-composition-ratio layer 115 and the surface relief
structure 150 is high is materialized.
[0149] When the first etching region 170 is etched, the
high-Al-composition-ratio layer is not exposed, and hence
horizontal spread of the etching is not so important. Therefore,
the etching is not limited to dry etching and wet etching may also
be used.
[0150] When wet etching is used, an etching stop layer may be used
to control the depth of the first etching region 170.
[0151] More specifically, an etching stop layer is provided above
the high-Al-composition-ratio layer 115 such that the etching of
the first etching region stops at the etching stop layer.
[0152] As a result, the requirement that the first etching region
170 should not reach the high-Al-composition-ratio layer 115 can be
satisfied with ease.
[0153] For example, an etchant used in etching the first etching
region 170 may be a phosphoric acid based etchant and, as the first
etching stop layer, for example, an AlInP layer at a thickness of
50 to 100 nm may be used.
[0154] In etching the second etching region 172, it is desirable
that the side surface of the high-Al-composition-ratio layer 115 is
exposed at a position which is substantially at an edge of the
patterning.
[0155] Whether the second etching region 172 is etched by dry
etching or wet etching, it is desirable that the distance from the
position at which the etching of the second etching region 172
starts to the high-Al-composition-ratio layer 115 be as short as
possible.
[0156] When the distance from the position at which the etching of
the second etching region 172 starts to the
high-Al-composition-ratio layer 115 is short enough, the position
of the side surface of the exposed high-Al-composition-ratio layer
115 can be sufficiently controlled also by using isotropic wet
etching.
[0157] For example, when the amount of misalignment between the
center of the aperture in the current confining structure 116 and
the center of the surface relief structure 150 is required to be
0.4 .mu.m or less, it is sufficient that the above-mentioned
distance is set to be 0.4 .mu.m or less.
[0158] On the other hand, when, as in a conventional case, the
etching starts from the uppermost portion of the upper mirror 114
to expose the high-Al-composition-ratio layer 115, the etching of
about 3 .mu.m is required, but it is difficult to align the center
of the surface relief structure 150 and the center of the aperture
in the current confining structure 116 with that precision.
[0159] When the second etching region 172 is etched by wet etching,
by using an etching stop layer similarly to the case of the etching
of the first etching region 170, the depth of the etching can be
controlled.
[0160] Here, the second etching region 172 may be etched using two
kinds of etchants. On the other hand, when the second etching
region 172 is etched using dry etching in which the amount of
vertical etching is large, the position of the edge of the
patterning and the position of the side surface of the
high-Al-composition-ratio layer 115 can be aligned with higher
precision.
[0161] Next, a specific example of using wet etching when the first
etching region 170 and the second etching region 172 are etched is
described.
[0162] FIG. 3 illustrates a layer structure for describing the
specific example.
[0163] As illustrated in the layer structure of the specific
example in FIG. 3, a first etching stop layer 310 which is an
AlGaInP layer is provided above the high-Al-composition-ratio layer
115 which is an AlAs layer while a second etching stop layer 320
which is made of AlGaInP is provided below the
high-Al-composition-ratio layer 115.
[0164] The first etching stop layer 310 is sandwiched in the upper
mirror 114 which is an AlGaAs multilayer reflecting mirror.
[0165] An etchant used in etching the first etching region 170 is,
for example, a phosphoric acid based etchant which is, for example,
a mixture of phosphoric acid, a hydrogen peroxide aqueous solution
(31%), and water with the volume ratio of the phosphoric acid, the
hydrogen peroxide aqueous solution, and the water being 4:1:90.
[0166] The etchant etches an AlGaAs semiconductor while the etchant
hardly etches an AlGaInP layer.
[0167] Therefore, the etching of the first etching region 170 stops
when the etching reaches the first etching stop layer 310.
[0168] In etching the second etching region 172, first, the first
etching stop layer 310 is removed using buffered hydrofluoric acid,
and then, the remaining upper mirror 114 and the
high-Al-composition-ratio layer 115 are etched using the
above-mentioned phosphoric acid based etchant.
[0169] The etching of the second etching region 172 stops when the
etching reaches the second etching stop layer 320.
Embodiment 3
[0170] Next, as Embodiment 3, a method of manufacturing a surface
emitting laser which is different from the above-mentioned
manufacturing method of Embodiment 2 in that a surface relief
structure is formed after a current confining structure is formed
is described.
[0171] FIGS. 4A to 4I illustrate manufacturing steps of the surface
emitting laser according to this embodiment.
[0172] In FIGS. 4A to 4I, like reference numerals designate like or
identical members illustrated in FIGS. 2A to 2K in Embodiment
2.
[0173] The method of manufacturing the surface emitting laser
according to this embodiment is different from the above-mentioned
manufacturing method of Embodiment 2 in that the surface relief
structure 150 is formed after the current confining structure 116
is formed while, in the manufacturing method of Embodiment 2, the
surface relief structure 150 is formed before the current confining
structure 116 described above is formed.
[0174] The manufacturing method according to this embodiment is
advantageous in that, compared with the manufacturing method of
Embodiment 2, the number of steps is smaller.
[0175] In the method of manufacturing the surface emitting laser
according to this embodiment, in steps illustrated in FIGS. 4A to
4C, similarly to the case of the manufacturing method of Embodiment
2, the layers are made to grow sequentially on the substrate.
[0176] More specifically, in the step illustrated in FIG. 4A, the
lower mirror 106, the lower spacer layer 108, the active layer 110,
the upper spacer layer 112, the high-Al-composition-ratio layer
115, and the upper mirror 114 are made to grow on the n-type GaAs
substrate 104.
[0177] Here, the uppermost portion of the upper mirror 114 forms a
GaAs contact layer having a thickness of 20 nm or more.
[0178] Then, in the step illustrated in FIG. 4B, the first
protective layer 260 is formed. After the first resist 262 is
applied, the mesa structure is patterned.
[0179] After that, in the step illustrated in FIG. 4C, the first
protective layer 260 is wet etched with the first resist 262 being
used as a mask.
[0180] The upper mirror 114 is etched with the first resist 262 and
the first protective layer 260 being used as a mask.
[0181] The steps illustrated in FIGS. 4A to 4C are the same as the
steps illustrated in FIGS. 2A to 2C, and thus, detailed description
thereof is omitted.
[0182] Then, in a step illustrated in FIG. 4D, the second resist
266 is applied, and patterning for forming the surface relief
structure 150 and an etching region (second etching region 172) for
exposing the side wall of the high-Al-composition-ratio layer 115
is performed.
[0183] The second resist 266 is patterned in the same way as the
second resist is patterned in Embodiment 2.
[0184] Then, in a step illustrated in FIG. 4E, the second etching
region 172 is etched with the second resist 266 being used as a
mask.
[0185] The etching is, for example, wet etching, and an etchant
used in the etching is, for example, a phosphoric acid based
etchant.
[0186] Then, in a step illustrated in FIG. 4F, the first protective
layer 260 is etched with the second resist 266 being used as a
mask. The etching is, for example, wet etching. An etchant which
hardly etches the GaAs contact layer which is the uppermost layer
of the upper mirror 114 is used. An example of the etchant includes
buffered hydrofluoric acid.
[0187] Through the steps illustrated in FIGS. 4E and 4F, the side
wall of the high-Al-composition-ratio layer 115 is exposed.
[0188] Further, it is desirable that the GaAs contact layer which
is the uppermost layer of the upper mirror 114 is hardly
damaged.
[0189] Then, in a step illustrated in FIG. 4G, after the second
resist 266 is removed, a part of the high-Al-composition-ratio
layer 115 is oxidized to form the current confining structure
116.
[0190] The second resist 266 is removed by, for example, acetone.
The high-Al-composition-ratio layer 115 is oxidized by, for
example, heating the high-Al-composition-ratio layer 115 to
400.degree. C. and placing the high-Al-composition-ratio layer 115
in a water vapor atmosphere for 30 minutes.
[0191] The high-Al-composition-ratio layer 115 is oxidized from its
side surface which is exposed to the second etching region 172. The
diameter of the aperture in the current confining structure 116
formed by the oxidation is, for example, 6 .mu.m. It is to be noted
that a part of the GaAs layer which is the uppermost layer of the
upper mirror 114 is exposed to the water vapor atmosphere, but it
is hardly oxidized at 500.degree. C. or lower.
[0192] Then, in a step illustrated in FIG. 4H, the upper portion of
the upper mirror 114 is etched with the first protective layer 260
being used as a mask to form the surface relief structure 150 in
the upper mirror 114.
[0193] The upper mirror 114 is etched by wet etching.
[0194] Then, in a step illustrated in FIG. 4I, the first protective
layer 260 is removed by, for example, buffered hydrofluoric
acid.
[0195] From hereon, similarly to the case of Embodiment 2,
according to a standard process, an electrode is connected to the
device to complete the surface emitting laser 100.
[0196] In the above embodiments, the second etching region 172 is
formed so as to be circular and surround the mesa structure, but it
is not necessarily required to be circular. Further, the second
etching region 172 may be divided into multiple parts.
[0197] For example, in a specific case of oxidizing a
high-Al-composition-ratio layer high in Al composition ratio, such
as an AlAs layer, in order to obtain a circular shape of the
current confining aperture, the second etching region 172 may be
disposed taking into consideration the in-plane anisotropy of the
speed of the oxidation.
[0198] The same can be said of a case, for example, of oxidizing a
high-Al-composition-ratio layer when a tilted substrate is
used.
[0199] In order to obtain a circular shape of the current confining
aperture, the second etching region 172 may be disposed taking into
consideration the in-plane anisotropy of the speed of the
oxidation.
[0200] According to the present invention, because the depth of the
etching of the second etching region 172 can be made shallow, the
second etching region can be disposed according to a predetermined
pattern with higher precision, and thus, the effect of the surface
relief structure can be produced with higher reliability.
[0201] Further, the mesa structure may be circular but it is not
essential.
[0202] Further, the materials of the semiconductor, the electrodes,
the dielectric, and the like, are not limited to the ones disclosed
herein and other materials may also be used within the gist of the
present invention.
[0203] Further, with regard to the method of manufacturing the
surface emitting laser, other steps may be added to or may replace
the steps disclosed herein which fall within the scope of the
present invention. For example, a cleaning step and the like may be
inserted.
[0204] Further, the etchant for etching the semiconductor and the
dielectric disclosed in the above embodiments may include other
etchants than the one disclosed in the above embodiments as long as
the other etchants fall within the scope of the present
invention.
[0205] Still further, in the above embodiments, with regard to the
mode control by the surface relief structure, a zero-order
transverse mode is made to be a single mode, and thus, the relief
structure has two regions, namely, a center portion of a light
emitting region having a high reflectivity, and a peripheral
portion thereof having a low reflectivity.
[0206] However, it is also possible to use the surface relief
structure to suppress zero-order mode oscillation and to make the
surface emitting laser oscillate in a specific high order mode. The
surface relief structure may have various shapes and sizes in order
to obtain a surface emitting laser having desired optical
characteristics.
[0207] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0208] This application claims the benefit of Japanese Patent
Application No. 2008-247737, filed Sep. 26, 2008, which is hereby
incorporated by reference herein in its entirety.
* * * * *